Means for forming SOI

ABSTRACT

A method for forming a SOI structure in which porous silicon is sealed and an epitaxial layer is grown thereover, followed by implantation of oxygen and annealing.

This application claims priority under 35 USC § 119(e)(1) of provisionalapplications No. 60/259,400 filed Dec. 30, 2000.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to integrated circuit structures andfabrication methods, and particularly to formation ofsilicon-on-insulator wafers.

BACKGROUND

Silicon on insulator (SOI) devices are important additions to ICtechnology for many reasons, allowing isolation of devices, low power,and low voltage technologies, as well as reducing parasitic capacitanceand the accompanying latch-up. Many technologies have been developed forthe fabrication of SOI devices, including separation by implanted oxygenand bonded wafers.

Porous silicon has also been used to form SOI by bonding wafers andlater removing the porous layer, leaving an epi layer with an oxide(which was originally grown on the epi by conventional means, andsandwiched by another wafer). The porous silicon in such a process isused as an etch-stop for the BOX.

Means for Forming SOI

The present application discloses an innovative way to make a SOI wafer.In the preferred embodiment, a wafer surface is anodized to form aporous silicon layer at the surface, which is sealed, preferably byheating it in an H2 ambient, after cleaning to remove any surface oxide.A thin silicon film is then formed by standard deposition techniques.The porous silicon is then implanted with a low dose of oxygen, followedby a high temperature anneal. The oxygen implant forms the buried oxidelayer. Additional epi layers may be formed on the layer ofmonocrystalline silicon on the surface.

Advantages of the disclosed methods and structures, in variousembodiments, can include one or more of the following:

-   porosity of silicon prevents clumping (formation of clusters rather    than a uniform layer) of SiO2 with low dose;-   relatively thick dielectric layer for given dose size;-   aids in planarity of SPIMOX;-   porous silicon readily oxidizes, allowing sharp definition of oxide    layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1 shows a flow chart of the preferred embodiment.

FIGS. 2 a–2 c show the formation of the SOI structure at differentprocess steps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

FIG. 1 shows a series of steps for practicing the preferred embodiment.First a layer of porous silicon is formed by anodizing a boron doped(0.001–0.02 ohm-cm) silicon wafer (step 1). In the preferred embodiment,a HF+C2H5OH combination is used. The depth of the porous silicon can becontrolled by timing or by limiting the depth of the boron doping.Thicknesses in the range of nanometers to microns can be obtained. Nextthe porous silicon surface is cleaned by diluted HF and then bydeionized water in order to remove only the wafer surface oxide. Next anepitaxial layer is grown on the porous silicon surface (step 2). Theporous layer is then implanted with oxygen using a plasma oxygen implantor other oxygen implantation methods (step 3).

The present application teaches that the porous silicon can be coveredwith a layer of epi and converted to an oxide by introducing oxygen intothe porous silicon. At least a thin layer of epi should be formed priorto oxidation of the porous silicon. Additional epi layers may be addedlater.

FIGS. 2 a–2 c show the innovative formation of the SOI during variousfabrication steps. FIG. 2 a shows the wafer substrate 202 afteranodizing. The top surface of the wafer exhibits cracks extending intothe depth of the surface resulting from the anodizing process. This areaof the wafer comprises the porous silicon 204. The porous silicon 204 istreated by a prebake to fill up some of the surface pores with siliconatoms that migrate toward the pores in order to reduce the surfaceenergy of the wafer.

FIG. 2 b shows the wafer 202 and porous silicon 204 covered by a thinlayer of epi 206. The quality of the epi 206 depends on the surface porefilling during the sealing bake of the wafer in a nitrogen, hydrogen, orinert ambient. A thin layer of epi used as a seal is required to give astarting point for later epi growth.

FIG. 2 c shows the wafer 202 and porous silicon 204 with the epi layer206. Oxygen 208 has been implanted into the porous silicon through theepi layer. Note that the oxygen implantation could be performed beforethe final epi formation. Oxygen doses on the order of 10^17 to 10^18oxygen ions per cm^2 are used in the preferred embodiment. Though 10^18is a relatively heavy dose, this may be done cheaply with plasmaimplantation into the BOX. High temperatures are used during oxidation,on the order of 1000 C for about 30 minutes.

Another possible process flow grows the epi layer over the poroussilicon before the oxygen is implanted. As another variation, a standardbut low dose oxygen implant can be used instead of using SPIMOX.

The porous silicon surface, after it has been formed but before oxygenhas been implanted or epi has been grown, can be sealed in an inertambient, a nitrogen ambient, or a hydrogen ambient, depending on theprocess.

According to a disclosed class of innovative embodiments, there isprovided: A method of forming a semiconductor-on-insulator structure,comprising the steps of: a) forming a structure having poroussemiconductor material at a first surface thereof; b) introducing anoxidizing species into said porous semiconductor material; and, eitherbefore or after step b), c) forming an epitaxial semiconductor layer onsaid porous semiconductor material, and reacting said oxidizing specieswith said porous semiconductor material to form a buried dielectriclayer beneath said epitaxial layer.

According to another disclosed class of innovative embodiments, there isprovided: A method of forming a semiconductor-on-insulator structure,comprising the steps of: a) anodizing a silicon wafer to form poroussilicon; b) introducing oxygen into said porous silicon; and, eitherbefore or after step b), c) forming a semiconductor layer on said poroussilicon, and reacting said oxygen with said porous semiconductormaterial to form a buried oxide layer.

According to another disclosed class of innovative embodiments, there isprovided: A method of forming a semiconductor-on-insulator structure,comprising the steps of: a) partially anodizing a silicon wafer to formporous silicon; and thereafter b) forming an epitaxial semiconductorlayer on said porous silicon; and thereafter c) introducing oxygen intosaid porous silicon, and reacting said oxygen with said porous siliconto form a buried oxide layer.

MODIFICATIONS AND VARIATIONS

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given, but is only defined by the issued claims.

Impurities can be incorporated into the BOX to alter the characteristicsof the device.

It is also contemplated that Si/Ge layers can be grown over aporous-silicon-derived oxide layer.

Also, in alternative embodiments the porous silicon does not have to becompletely oxidized.

In another class of alternative embodiments, it is contemplated that theoxidizing species does not have to be derived from molecular oxygen, butcan be derived from oxidizing gasses such as O3 or N2O.

In another class of alternative embodiments, it is contemplated thatother elements besides oxygen can be used for the self-segregatingreaction which forms the buried layer.

The teachings above are not necessarily strictly limited to silicon. Inalternative embodiments, it is contemplated that these teachings canalso be applied to structures and methods using other semiconductors,such as silicon/germanium and related alloys, gallium arsenide andrelated compounds and alloys, indium phosphide and related compounds,and other semiconductors, including layered heterogeneous structures.

Additional general background, which help to show the knowledge of thoseskilled in the art regarding variations and implementations of thedisclosed inventions, may be found in the following documents, all ofwhich are hereby incorporated by reference: Coburn, PLASMA ETCHING ANDREACTIVE ION ETCHING (1982); HANDBOOK OF PLASMA PROCESSING TECHNOLOGY(ed. Rossnagel); PLASMA ETCHING (ed. Manos and Flamm 1989); PLASMAPROCESSING (ed. Dieleman et al. 1982); Schmitz, CVD OF TUNGSTEN ANDTUNGSTEN SILICIDES FOR VLSI/ULSI APPLICATIONS (1992); METALLIZATION ANDMETAL-SEMICONDUCTOR INTERFACES (ed. Batra 1989); VLSI METALLIZATION:PHYSICS AND TECHNOLOGIES (ed. Shenai 1991); Murarka, METALLIZATIONTHEORY AND PRACTICE FOR VLSI AND ULSI (1993); HANDBOOK OF MULTILEVELMETALLIZATION FOR INTEGRATED CIRCUITS (ed. Wilson et al. 1993); Rao,MULTILEVEL INTERCONNECT TECHNOLOGY (1993); CHEMICAL VAPOR DEPOSITION(ed. M. L. Hitchman 1993); and the semiannual conference proceedings ofthe Electrochemical Society on plasma processing; Current Progress inEpitaxial Layer Transfer, Sakaguchi, et al., IEICE Transactions, Vol.E80-C, No.3, March 1997; Epitaxial Layer Transfer by Bond and Etchbackof Porous Silicon, Yonehara, et al., Appl. Phys. Lett. 64 (16), 18 Apr.1994.

1. A method of forming a semiconductor-on-insulator structure,comprising the steps of: a) forming a structure having poroussemiconductor material at a first surface thereof; b) sealing saidsurface; c) forming an epitaxial semiconductor layer on said poroussemiconductor material after said sealing; d) implanting an oxidizingspecies through said epitaxial layer into said porous semiconductormaterial;and reacting said oxidizing species with said poroussemiconductor material to form a buried dielectric layer beneath saidepitaxial layer.
 2. The method of claim 1, wherein said oxidizingspecies consists essentially of oxygen.
 3. The method of claim 1,wherein said semiconductor layer consists essentially of silicon.
 4. Amethod of forming a semiconductor-on-insulator structure, comprising thesteps of: a) anodizing a silicon wafer to form porous silicon; b)sealing said surface; c) forming a semiconductor layer on said poroussilicon after said sealing; d) implanting an oxidizing species throughsaid epitaxial layer into said porous semiconductor material; and e)reacting said oxygen with said porous semiconductor material to form aburied oxide layer.
 5. The method of claim 4, wherein said semiconductorlayer consists essentially of silicon.
 6. A method of forming asemiconductor-on-insulator structure, comprising the steps of: a)partially anodizing a silicon wafer to form porous silicon; andthereafter b) sealing said surface; c) forming an epitaxialsemiconductor layer on said porous silicon; d) implanting oxygen intosaid porous silicon through said epitaxial semiconductor layer; and e)reacting said oxygen with said porous silicon to form a buried oxidelayer.
 7. The method of claim 6, wherein said oxidizing species consistsessentially of oxygen.
 8. The integrated circuit of claim 6, whereinsaid semiconductor layer consists essentially of silicon.
 9. A productmade by the process of claim
 1. 10. A product made by the process ofclaim
 4. 11. A product made by the process of claim
 6. 12. The method ofclaim 1 wherein said step of sealing includes heating said poroussemiconductor material in a hydrogen ambient.
 13. The method of claim 4wherein said step of sealing includes heating said porous semiconductormaterial in a hydrogen ambient.
 14. The method of claim 6 wherein saidstep of sealing includes heating said porous semiconductor material in ahydrogen ambient.